Overturning mistaken decisions is an occasional duty of the Supreme Court, whose noblest achievement was the protracted, piecemeal repudiation, with Brown v. Board of Education (1954) and subsequent decisions, of its 1896 ruling that segregated "separate but equal" public facilities were constitutional. This Monday, the court will hear oral arguments that probably will presage another overdue correction.

The issue is: Are Mark Janus' First Amendment rights of freedom of speech and association (which entails the freedom not to associate) violated when government requires him, an employee, to pay "fair share" or "agency" fees to a private entity, a labor union, to which government has given exclusive power to represent him, although he chooses not to be a member? Janus argues that an exclusive representative "is indistinguishable from a government-appointed lobbyist." The fees are usually significantly more than half of — sometimes up to 100 percent of — union dues.

In its 1977 Abood decision, the court upheld such exactions. But the ruling contained the seeds of its coming — by this June — reversal, because it acknowledged this: "There can be no quarrel with the truism that, because public employee unions attempt to influence governmental policymaking, their activities . . . may be properly termed political." And in a concurring opinion, Justice Lewis Powell noted that "the ultimate objective of a union in the public sector, like that of a political party, is to influence public decision-making." So Abood made compulsory political contributions constitutional.

For 41 years, the court has advanced the slow-motion undoing of Abood with decisions subjecting various instances of compelled speech to strict scrutiny. For example, in 1983 it held that "speech on public issues occupies the 'highest rung of the hierarchy of First Amendment values.'" In 2014, the court said it is a "bedrock principle that, except perhaps in the rarest of circumstances, no person in this country may be compelled to subsidize speech by a third party that he or she does not wish to support."

Abood ignored the inherently political nature of bargaining by government employees' unions. In America's 27 right-to-work states, employees cannot be forced to join a union as a condition of employment. In the other 23, including Janus' Illinois, workers must join a union or pay fees. The supposed constitutionality of this compelled speech rests on the fiction that these fees pay only the costs of collective bargaining, from which the fee payers benefit.

In private-sector collective bargaining, management and labor negotiate about how to distribute companies' profits. No comparably adversarial process exists in the public sector. There government, which acquires its "profits" (revenues) from a third party — taxpayers — "negotiates" with unions that have an interest in government doing what it wants to do anyway: expand. Because government is both employer and policymaker, collective bargaining by the union is inherently political advocacy and indistinguishable from lobbying. Hence in 2012 the court acknowledged that compulsory fees are "compelled speech and association" implicating on First Amendment rights. President Franklin Roosevelt was right: "The process of collective bargaining, as usually understood, cannot be transplanted into the public service."

Although organized labor's portion of the private-sector workforce has plummeted from around 35 percent in 1953 to 6.5 percent today, it now organizes about one-third of local, state and national government workers. Organized labor now is primarily governments organized as interest groups.

Union officials' salaries and benefits are the biggest expense of the union that Janus is forced to finance (the American Federation of State, County, and Municipal Employees). It is facially implausible that most of what these officials do is devoted exclusively to collective bargaining and is hermetically sealed from AFSCME's aggressive promotion of its broad political agenda. Besides, money is fungible: Money extracted from reluctant nonmembers can fund activities that otherwise would have been paid for with money that now can be devoted to other political causes.

And AFSCME's approximately 3,400 local unions calculate their own supposed "collective bargaining" allocations. So each enjoys vast discretion in deciding which of its expenditures are germane to collective bargaining, and therefore what their nonmembers owe to the union.

Many Democrats are, or say they are, distraught about "big money" and there being "too much" money in political campaigns. They will, however, be seriously distraught if help arrives by June with Janus winning. This will stop the coerced flow of money to government workers' unions, which in the 2016 election cycle spent $63.9 million on politics, 90 percent supporting Democratic candidates and causes.

Disclosure: This columnist's wife, Mari Will, is an adviser to Illinois Gov. Bruce Rauner, who originally filed the lawsuit that became Janus v. AFSCME.

George F. Will is one of today's most recognized writers, with more than 450 newspapers, a Newsweek column, and his appearances as a political commentator on Fox news. Read more reports from George Will — Click Here Now.